1. Field of the Invention
The present invention relates to a brake drum for a wet-type band brake which is used in an automatic transmission, and a method for manufacturing the same.
2. Related Background Art
An automatic transmission for a vehicle often employs a wet-type band brake in order to control gear shift elements. A wet-type band brake usually includes a brake band which is made of a single steel plate annularly formed with a frictional member bonded onto the inner peripheral surface thereof. The diameter of this brake band is contracted by an actuator so as to fasten a brake drum disposed inside.
The brake drum for a wet-type band brake comprises an outer cylinder with a slide contact surface which is in a slide contact with the frictional member of the brake band and an inner cylinder serving as a bearing portion. The wet-type band brake is generally used for gear shift control and is usually required to have a sufficient braking performance. For this reason, in order to enhance the braking power at an initial stage of the frictional engagement, such a brake band is conventionally present which is provided with small grooves (annular grooves or helical grooves) substantially along the circumferential direction of the slide contact surface of the brake drum so as to increase a coefficient of friction thereof (see, e.g., Japanese Patent Application Laid-Open No. 8-177903 (official gazette, p. 4, FIG. 2), and Japanese Patent Application Laid-Open No. 11-51097 (official gazette, p. 2, FIG. 1)).
In a wet-type band brake of this type is, as shown in FIG. 11, for example, grooves 5 are formed at a comparatively large pitch on a slide contact surface 2 of a brake drum by cutting work. For this reason, a land 7 between each adjacent grooves 5 and 5 was not mechined and has a cross section in a linear form. Then, an edge 9 is formed in a border between the groove 5 and the land 7.
However, if the axial dimension of the land 7 is large, an oil film is interposed between the land 7 and the brake band so that a coefficient of friction at the initial stage of the frictional engagement can not be increased to a predetermined value. Particularly, since the viscosity of the transmission oil is increased at a cooling time, or the like, the oil film is hardly broken when the brake band comes into slide contact, thereby lowering the coefficient of friction. As a result, there arises a problem that the gear shift control in the automatic transmission is difficult to be conducted ideally. Note that the edge 9 which is formed in the border between the groove 5 and the land 7 may damage a little the frictional member of the brake band which is formed of a comparatively soft material at the time of frictional engagement.
In order to prevent the coefficient of friction from lowering at the initial stage of frictional engagement, it is required to reduce the axial dimension of the land 7 in order to reduce the oil film which is interposed between the land 7 and the brake band. Thus, it is examined to reduce the pitch between the grooves 5 and 5.
However, when the pitch between the groves 5 and 5 is reduced to be smaller than the width of the tip end portion of a machining tool (for example, a cutting tool) to eliminate the land, as shown in FIG. 12, the edge 9 formed between the grooves 5 and 5 becomes very sharp, thereby disadvantageously damaging the frictional member of the brake band to the extent which can not bear comparison with that with the land 7 shown in FIG. 11. The present inventors, et al., have examined to remove the edge 9 by grinding. However, this method would result in an increased number of machining or processing steps and a higher machining or processing cost, so as to deteriorate the mass productivity. As a result, it is difficult to employ this method practically.